Mixotrophic microorganism, such as microalgae and cyanobacteria, may be cultivated to produce proteins, lipids, carbohydrates, pigments, and polysaccharides that are useful in a variety of products. The metabolism of mixotrophic microorganisms includes both phototrophic and heterotrophic metabolisms to utilize multiple inputs as energy and carbon for growth and production of gases. When light is utilized as an energy source and inorganic carbon (e.g., carbon dioxide) is utilized as a carbon source by the phototrophic metabolism for growth, the microorganism's net oxygen production is positive and carbon dioxide is consumed. When an organic carbon source is utilized as both an energy source and a carbon source by the heterotrophic metabolism for growth, the microorganism's net carbon dioxide production is positive and oxygen is consumed. With the production and consumption of oxygen and carbon dioxide alternating or happening simultaneously within a mixotrophic microorganism, the composition of gasses saturated in the culture medium is continually changing.
Using conventional air sparging and carbon dioxide injection in a mixotrophic culture does not maintain a correct balance of gases for efficient growth in a continually changing culture. The gas imbalance created by conventional air sparging and carbon dioxide injection in a mixotrophic culture may result in gases being lost to the atmosphere before utilization by the microorganisms in the aqueous medium, which reduces the method efficiency and wastes resources. Toxic saturation of a gas in the culture medium may also occur, which may provide an inhibiting effect on the growth of the mixotrophic microorganisms. The use of air sparging may also create foam (comprised of proteins, polysaccharides, carbon and other organics), which can facilitate the proliferation of contaminating organisms (e.g., bacteria, fungi and predators) by harboring the contaminating organisms and providing a feed source for the contaminating organisms. The foam may also block or limit light available for photosynthesis. Both the facilitation of contaminating organisms and blocking of light may inhibit the growth of the mixotrophic microorganisms. Another drawback of systems using conventional air sparging and carbon dioxide injection is the need to supply gasses from an external source to bioreactors, which may reduce the length of each run of a bioreactor circulation path or require additional gas injection points, thus potentially limiting the volumetric capacity of the bioreactor. The additional gas injection points may also increase the costs of constructing and operating the bioreactor.
Therefore, there is a need in the art for a method of culturing mixotrophic microorganisms which efficiently utilizes carbon dioxide and oxygen, and reduces the accumulation of foam in the culture.